1. Field of the Invention
The present invention relates to switching power supply circuits, and more particularly, to a current-resonant switching power supply which is effective when an output voltage is obtained at the secondary side in a synchronous rectifying method.
2. Description of the Related Art
As the use of less energy has been required for Earth environmental protection in recent years, higher efficiency and lower noise are further demanded for various types of switching power supplies.
As power supply circuits for computers and communication units, DC--DC converters which have low noise and which maintain high efficiency even at a low-voltage output are required.
When a low voltage is output, a high output current is generally obtained if a constant power consumption is maintained. In a DC--DC converter, a resistance loss at a rectifying diode in the secondary side becomes a large power loss.
Therefore, it is considered that a current-resonant switching power supply having relatively low noise and high efficiency and a transistor which provides a low on-resistance at the secondary-side output are driven in a synchronous rectifying method to obtain a DC output voltage.
FIG. 4 shows a switching power supply circuit having such a combination. Switching devices Q1 and Q2 are formed of MOS FETs connected in series, and an insulating transformer T transfers a switching power at the primary side to the secondary side.
A power supply control circuit IC alternately opens and closes the switching devices Q1 and Q2, and is usually configured such that a reference voltage is compared with an output voltage V.sub.0 by voltage detection means (not shown) to adjust the switching frequency of the switching devices in order to obtain a constant output voltage V.sub.0.
The outputs of the switching devices Q1 and Q2 are sent to a primary winding L.sub.1 of the insulating transformer T and a resonant capacitor C.sub.1. When the switching devices Q1 and Q2 are alternately opened and closed, the primary winding L.sub.1 of the transformer is driven by a current that charges and discharges the resonant capacitor C.sub.1, which resonates with the leakage inductance of the transformer T. As shown in FIG. 5, a voltage V.sub.1 applied to the primary winding L.sub.1 causes an induced voltage V.sub.2 at the secondary winding L.sub.2, and full-wave rectification is applied by one set of rectifying diodes in a usual DC--DC converter.
Since a loss caused by the rectifying diodes, which have relatively high on-resistances, is rather large when an output voltage is low, there is known a circuit in which N-channel MOS transistors Q3 and Q4 are used instead of the rectifying diodes and full-wave rectification is applied in a synchronous method to output the DC voltage V.sub.0 from a smoothing capacitor C.sub.0, as shown in FIG. 4.
In the circuit shown in FIG. 4, since a full-wave-rectified voltage is accumulated in the smoothing capacitor C.sub.0 through the MOS transistors Q3 and Q4 at a low resistance, a relatively low DC voltage V.sub.0 can be efficiently output.
Parasitic diodes D are formed due to the structures of the MOS transistors Q3 and Q4.
A current-resonant switching power supply, which has half-bridge connected switching devices, features essential low noise with zero-current switching at turning on and current resonance at turning off, and a wide variable output voltage V.sub.0 at the secondary side with a switching frequency being changed. The power supply has in the entire period a rectified current continuous mode in which power is transferred to the secondary side and a secondary-side rectification discontinuous mode in which power is not sent to the secondary side in a period, in order to provide a wide regulation range.
When the switching frequency becomes lower than the resonant frequency due to constant-voltage control, the mode is changed to the secondary-side rectification discontinuous mode. In this case, the rectifying capacitor at the secondary side is not charged for periods t1 in one switching cycle as shown in FIG. 5, and the output voltage V.sub.0 is higher than the secondary voltage V.sub.2 of the transformer in the periods t1.
Since a reverse current is blocked by diodes, even in such a discontinuous mode in a usual rectification with the diodes, there occurs no problem. In rectification with MOS FET transistors, however, since a reverse current flows, when the transistors are controlled such that they are turned on in the periods, negative-direction currents id1 and id2 flow into the synchronous-rectifying MOS transistors Q3 and Q4 in the periods t1 in the reverse direction, respectively, as shown in FIG. 5.
With these currents id flowing in the negative direction, the MOS transistors Q3 and Q4 generate heat and a switching loss occurs at the primary side.
To solve these problems, it has been considered that control circuits IC1 and IC2, which each have a logic circuit for detecting the output voltage and the current of the transformer T to control the MOS transistors Q3 and Q4, be provided such that the MOS FET transistors Q3 and Q4 are turned on at appropriate times. Since such control circuits IC1 and IC2 need to be prepared separately, however, the cost of a power supply increases and the circuit configuration is made complicated.
When a charging period in which the capacitor C.sub.0 is charged becomes short, the peak value of the current accumulated in this period becomes large and a continuity angle becomes small to reduce the power factor of the switching power supply.